Optical devices such as waveguides, filters, switches, and photonic circuits with a broadband optical response are highly desirable for advanced optical communication systems.1-3 A technique for generation of light over a wide spectral range is second harmonic generation (SHG). For efficient conversion, quasiphase matching is typically required whereby the relative phase difference between the interacting waves is corrected by using periodic structures. Two approaches include periodic poling of nonlinear media and formation of photonic crystals.4,5 The utilization of periodic structures, however, limits the performance to a narrow range of wavelengths. Recently, generation of broadband SHG, however, was demonstrated in disordered, nonlinear optical media using random quasiphase matching.6,7 
For efficient SHG, a material with a large second order nonlinear optical coefficient is required. Of the nonlinear optical materials, ferroelectric oxides are widely utilized due to their intrinsically high optical nonlinearities. While lithium niobate has been widely studied for SHG, other ferroelectrics have even larger nonlinear coefficients. In particular ferroelectric BaTiO3 (BTO) has been shown to have excellent nonlinear optical properties.8,9 Optically transparent epitaxial thin film waveguides of BTO are also now available. In addition, thin films potentially enable integration of optical components. Several groups have already demonstrated deposition of BTO thin films on Si substrates.10,11 By combining of BTO thin film deposition with Si semiconductor circuit processing, photonic circuits with rich functionality are expected.
SHG of light using BTO thin films has been previously demonstrated in the transmission mode where the pump light beam is normal to the film surface.8 An obvious limitation of transmission SHG using thin films is the short light interaction length resulting in low conversion efficiency.12 Another disadvantage of the transmission is that the SHG mode is not suitable for integration of optical components.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.